Field of the Invention
The invention relates to a leadframe and in particular to a leadframe constructed in such a way that it can be sheathed with plastic in an especially advantageous way, when joined together with a semiconductor chip to make a semiconductor module. The leadframe of the invention is especially suitable as a leadframe for DRAM (Dynamic Random Access Memory) components, and in particular those with a package or housing of the TSOP-II type (Thin Small Outline Package, type II). Such packages have a plastic thickness of only about 1 mm.
The DRAM components are typically mounted according to the LOC (Lead On Chip) technique, in which the semiconductor chip is secured under the leadframe, for instance with the aid of mounting tapes (adhesive film). Since DRAM chips become smaller with each shrinking generation, it often happens that the width of the DRAM chips is markedly less than the package width, or the width of the mold cavity with the aid of which the package is produced. Relatively large voids, into which the plastic composition can penetrate unhindered, thus occur in the mold cavity on a side of the leadframe connected to the semiconductor chip, in regions around the semiconductor chip. When the mold cavity is filled with plastic composition (during molding), it is therefore often observed that the plastic composition spreads faster in the voids next to the semiconductor chip than in the regions where the semiconductor chip is located. In other words, the plastic races ahead in the peripheral regions of the mold cavity and reaches the ventilation side faster than the remainder of the front of the flowing plastic. The plastic fronts extending to the right and left of the semiconductor chip meet at the end of the mold cavity before the plastic composition has completely filled the middle portion of the package, and therefore air is trapped in the region on the ventilation side, for instance behind or above the chip. The trapped air reduces the quality and reliability of the finished component. The quality loss can be so great that the component has to be discarded.
The air inclusion cannot be eliminated by compression in the sheathing process. In order to prevent air inclusions in the production of plastic packages, three different techniques which have previously typically been used are known as the "lost cavity", "vacuum molding" and "balanced package" techniques.
In the lost cavity technique, a further cavity, the so-called lost cavity, is disposed behind the mold cavity for the package. The lost cavity serves as a receiving space for any air inclusions that occur and that are forced out of the package void by additional plastic composition. The plastic composition forced into the lost cavity is thrown away as waste, which is a disadvantage from the standpoint of expense and environmental protection. Moreover, separating the lost cavity plastic body from the desired product requires additional method steps.
In vacuum molding, the mold cavity is evacuated with a vacuum device before being filled with the plastic composition, so that the plastic flows into a practically air-free space, and air inclusions are thus prevented. That method, however, entails considerable additional expense for equipment since, for instance, an air-tight seal of the molding tool has to be achieved.
A fundamentally different method is to construct the semiconductor module to be sheathed in such a way that a uniform flow of the plastic in the mold cavity is assured, and the inclusion of air is averted. Semiconductor modules of that kind, which assure a uniform mold flow, are generally called balanced packages. Balanced packages are achieved, for instance, through the use of a particular leadframe structure and exact adaptation of the chip thickness and chip length. In the case of DRAM components or semiconductor modules in general with a narrow chip and a wide package, balanced packages cannot be achieved by conventional structural provisions (such as a shallow leadframe without embossed features, bends or the like).